|The laptop works ok on mains only - no battery installed?|
I'm inclined to doubt the charger specs you have are correct... When I google for the charger specs. I find they are seriously different to what you quote.
If you run a google trawl/search using:
Dell precision m4600 charger specs
you'll get a whole slew of hits; and most (all?) of them suggest quite a different spec. - not the least the current rating being 6.7Amp - which is more typical of many laptop chargers? Your stated 3.34Amps seems unusually low based on my own (limited) experience.
is just one found using the above search words/string. Suggest you repeat my trawl and see what you find?